What's a crossover? Let me guess: you're a drummer, right? :) OK, in a generic, global sense, crossovers are used to "direct" the frequencies of sound to the appropriate speaker.You don't want the lows going to the tweeters, or else the tweets are gonna blow up. Is that generic enough?
A "passive" crossover is typically used IN the speaker cabinet, and protects the HF driver from being blown up by the bass notes. It takes the high-power output from the power amp and "splits" it to the appropriate loudspeaker component. An "active" (or "electronic") crossover works on the relatively low-power output from the mixing board, AHEAD of the power amp(s), and directs the frequencies to the proper amp/speaker configuration. If you are using a low frequency amp and a high frequency amp, each directly driving the appropriate speaker, this is called "bi-amping". If you split it 3 ways instead of 2, it is called "tri-amping". This arrangement tends to deliver cleaner sound to the speakers because the bass notes are going to the amp that's also driving the HF speakers. This results in a cleaner, more efficient sound.
But all this is moot if you don't know what you're doing. And, please don't take offense at this, but if you don't know this simple theory of operation, you don't know what you're doing. To correct this malady and to keep from being ripped off by GC sales personnel and from BLOWING UP your rig, you need to read up on this stuff. Go to the MIX Bookshelf and get the Sound Reinforcement Handbook. Also, check out the Sound Systems for Houses of Worship (I think that's the title). They are both listed in the "Live Sound" section. These will get you a LOT farther than anything else.

that really cleared me up. I working on putting together a list for a pa system for my band. And I was going through Musiciansfriend. and i was looking at crossovers, but the only problem was, i had no idea if i needed one or not, cause i had no idea what their purpose was. right now our buget will be really tight for getting the pa. around $1200. we already have a mixer, and a bunch of mics. and what i plan on getting is just a simple set up of a power amp and 2 yamaha 2clubv112s or 115s, might just stick to the 12s. then for the future i plan on adding on 2clubv115s subs.

OK. You WILL need to have an "electronic crossover" and a seperate power amp to drive the subs. The FOH speakers you listed will do OK
with their own internal passive crossovers for the time being. Be careful with the power amp. My suggestion for that budget is a Peavey CS800 or higher. Peavey is NEVER my first choice for gear, but they DO have a proven track record in that field. There are tons of amp manufacturers out there these days, some big names with Chinese guts. Peavey and Yamaha are, for the most part, rocks of stability in this regard. Good luck.

while it's kinda pricey at first glance... i'ld think about looking at a dbx driverack.. it's got your crossover/compressor/EQ/feedback controller all in one .... and the price for good amps have come down quite a bit int the last few years... check out the lower end crown or qsc... they'll be around a long time...

for the buget that i'm on, the driveracks are too pricey, but yes, i am looking into them for the future once i add monitors and subs into the mix

Also, compression/gateing. Should it be done straight outta the mic? or since i'm planning on using the pa just for vocals, and possibly just a keyboard, would i be able to use it right before the power amp?

In your particular case, you might want a compressor to be used as a "limiter" on the whole house mix, but you will have to learn to set it properly. A compressor can be very useful in live sound, but it can be a destructive tool as well. If you are renting out the system, you HAVE to protect it from being mis-used and overdriven. A compressor, set at a fairly high threshold to act as a peak limiter, can act as a "safety valve" to keep from overdriving the power amps.
Instead of buying some POC from Musicians' Fiend :wink: , try going on e-Bay and find a good, used PRO unit, like the dbx 166 or a Symetrix 501. I love the 501. It is a mono unit, but has a seperate peak limiter section from the compressor. This lets you set a little compression to rein in a dynamic singer, and still have a peak limiter to prevent blowing up the rig. I currently have 12 of these in my racks for various-sized live gigs. I use them predominantly on stage mixes. And to use 1 on your main mix, you would connect it between the main out from the mixer, before the signal hits your EQ and power amps...I typically have paid between $100-125 per 501. A great deal! If you end up getting one, let me know and I'll go over the proper settings and tricks to keep it from being a trainwreck on the mix.
And I mentioned EQ. There have been times when I get more bang from the limiter than EQ, but a decent equalizer for monitors and mains is very crucial....Got EQ?

at this point, the compressor is still just a thing for me to get in the future (btw i was planning on getting something dbx). right now i'm just working on getting the money together for the power amp and then speakers.

also, i wouldn't be just giving the pa to a band and let them have free run of it, i would be there to run it. (i'm not gonna spend that much money to have other people go a destroy my system)